Acid-labile isotope-coded extractant (ALICE) and its use in quantitative mass spectrometric analysis of protein mixtures

ABSTRACT

The method of the invention provides novel compounds, termed acid-labile isotope-coded extractants (ALICE), for quantitative mass spectrometric analysis of protein mixtures. The compounds contain a thiol-reactive group that is used to capture cysteine-containing peptides from all peptide mixtures, an acid-labile linker, and a non-biological polymer. One of the two acid-labile linkers is isotopically labeled and therefore enables the direct quantitation of peptides/proteins through mass spectrometric analysis. Because no functional proteins are required to capture peptides, a higher percentage of organic solvent can be used to solubilize the peptides, particularly hydrophobic peptides, through the binding, washing and eluting steps, thus permitting much better recover of peptides. Moreover, since the peptides are covalently linked to the non-biological polymer (ALICE), more stringent washing is allowed in order to completely remove non-specifically bound species. Finally, peptides captured by ALICE are readily eluted from the polymer support under mild acidic condition with high yield and permit the direct down stream mass spectrometric analysis without any further sample manipulation. In combination with our novel dual column two dimensional liquid chromatography- mass spectrometry (2D-LC-MS/MS) design, the ALICE procedure proves to a general approach for quantitative mass spectrometric analysis of protein mixtures with better dynamic range and sensitivity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the priority of U.S. ProvisionalPatent Application No. 60/242,643, filed Oct. 23, 2000.

BACKGROUND OF THE INVENTION

The present invention relates to the field of high-throughputquantitative protein analysis and, more specifically, to novel reagentsfor use in such analysis.

Most approaches to quantitative protein analysis are accomplished bycombining protein separation, most commonly by high-resolutiontwo-dimensional polyacrylamide gel electrophoresis (2D-PAGE), with massspectrometry (MS)-based sequence or tandem mass spectrometry(MS/MS)-based sequence identification of selected, separated proteinspecies.

S. P. Gygi, et al., Nature Biotech, 17:994-999 (October 1999) describesan approach to quantitative protein analysis based on a class ofreagents termed isotope-coded affinity tags (ICAT), which consist ofthree functional elements: a specific chemical reactivity, anisotopically coded linker, and an affinity tag. The reagents describedby Gygi utilize biotin as the affinity tag and rely upon biotin-avidinaffinity binding to isolate the cysteine-containing peptides from thecomplex peptide mixture.

Although the ICAT approach has many advantages over the traditional2D-PAGE/MS approaches, it does possess some intrinsic limitations. Forexample, ICAT adds a relatively large chemical moiety onto thecysteine-containing peptides and this functionality is very labile undercollision induced dissociation (CID) condition and thus complicates thedownstream data analysis. Non-specific binding is also a concern sincethe enrichment relies on non-covalent affinity binding between a protein(avidin) and the biotinylated peptides. Finally, the captured peptidesare not readily eluted from the avidin beads with high recovery usingMS-compatible conditions. Thus, there is a need in the art foradditional reagents and methods for improving performance inquantitative mass spectrometric analysis of protein mixtures.

SUMMARY OF THE INVENTION

The invention provides polymer-based compounds useful for quantitativeanalysis of mixtures containing proteins. Advantageously, the compoundsof the invention bind covalently with the peptides which they are usedto tag, permitting the tagged peptides to be subjected to more rigorouswashing techniques. Thus, the tagged peptides are more readily purified,without nonspecifically bound species. This results in lower backgroundon MS spectra and thus provides an increase of dynamic range andsensitivity in quantitation and identification of the proteins.

In one aspect, the invention provides a method for the quantitativeanalysis of mixtures containing proteins. The method involves (a)reducing the disulfide bonds in the proteins of a sample to provide freethiol groups in cysteine-containing proteins; (b) blocking free thiolson the reduced proteins with a blocking reagent; (c) digesting theproteins in the sample using an enzyme such as trypsin; (d) reducing thepeptides following the digestion step; (e) reacting cysteine-containingpeptides with a reagent, wherein the reagent comprises a thiol-specificreactive group covalently bound to a polymer tag via a linker, whereinthe linker can be differentially labeled with stable isotopes(optionally prior to or following any of the reduction steps); (f)washing the polymer-bound peptides to remove non-covalently boundcompounds; (g) eluting the cysteine-containing peptides; and (h)subjecting the retrieved peptides to quantitative mass spectrometry (MS)analysis. In one embodiment, the method further involves performingsteps (a) to (d) on a second sample; reacting cysteine-containingpeptides in the second sample with a stable isotope-labeled form of thereagent, wherein in reacting step (e), the reagent used is a non-isotopelabeled form of the reagent; mixing the peptides of the reacted samplefollowing step (e) and the reacted second sample; and performing steps(g) and (h) on the peptides in the mixture.

In another aspect, the invention provides a compound useful forcapturing cysteine-containing peptides. This compound is composed of athiol-specific reactive group attached to a non-biological polymer via alinker. In one desirable embodiment, the reagent has the formula:A1—Linker—A2—polymer, wherein A1 is a thiol-reactive group and A2 is anacid labile group to which the polymer is attached.

In yet another aspect, the invention provides a reagent kit for the massspectral analysis of proteins that comprises a compound of theinvention.

Other aspects and advantages of the present invention are describedfurther in the following detailed description of the preferredembodiments thereof

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A and FIG. 1B provides a schematic of the automated 2D-LC/MSSystem of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a novel approach for the quantitativeanalysis of proteins using acid-labile isotope coded extractants (ALICE)which are useful for capturing cysteine-containing peptides. Theadvantage of this approach over the prior art, is that it replacesbiotin-avidin affinity binding with acid-labile covalent binding toretrieve cysteine-containing peptides from the mixture. Since thebinding is covalent, more stringent detergents or organic solvents canbe used during the procedure to keep hydrophobic proteins and peptidesin the solution and thus maximize the overall peptide recovery.Furthermore, the compounds and method of the invention avoid nonspecificpeptide-protein binding. Removal of all detectable non-covalently boundspecies during the washing step(s) is also accomplished. Thus, the finalcysteine-containing peptide solution is much less contaminated,resulting in higher sensitivity and dynamic range of MS analysis.Lastly, since the ALICE label is small in size and does not undergofragmentation during MS/MS analysis, it does not interfere with thedownstream MS analysis and database searching.

In one embodiment, the present invention provides a compound of theformula: A1—Linker—A2—polymer, wherein A1 is a thiol-reactive group andA2 is an acid labile group to which the polymer is attached.Alternatively the acid labile group may be absent and the polymer may beattached directly to the linker.

Most preferably, the polymer is a non-biological polymer. As used hereina non-biological polymer includes inorganic polymers and organicpolymers which form a covalent bond with the acid-labile group, wherepresent, or the linker. Suitably, an organic polymer selected does notinterfere with the process steps in the method of the invention, e.g.,is stable under basic conditions and in the presence of the detergentsand/or organic solvents required to maintain the mixture in solution. Inone suitable embodiment, the polymer used in the invention is a solidsubstrate composed of a homopolymer or a heteropolymer containingpolystyrene, polyethylene, polyacrylamide, polyacrylein, polyethyleneglycol, or the like. Suitable polymers and solid substrates, e.g.,resins, beads or the like, are available from a variety of commercialsources including Sigma-Aldrich, NovaBiochem, and Beckman-Coulter, ormay be synthesized using known techniques. An example of one suitablesynthesis technique is provided in Example 1 below. However, theinvention is not so limited.

In one embodiment, the polymer is covalently bound to the linker via anacid-labile group that provides the compound of the invention with theability to be readily eluted using an acidic reagent. In one preferredembodiment, the acid-labile group bound to the polymer has the followingstricture:

in which the linker is —CONH—, —COO—, or another amide or ester.However, other structures can be readily synthesized to contain othersuitable groups that provide similar qualities to the compound in termsof stability and accessibility to acid elution. Examples of suitableacid-labile groups include:Rink Amide Linker:

DHP Linker:

Siber Linker:

Trityl Linker:

Wang Linker:

In certain embodiments, this function may be provided by the linker, andthe acid labile group may be absent.

The linker is any structure that may be differentially labeled withstable isotopes for use in MS techniques. In one embodiment, the linkercontain from 1 to 100 atoms in length, about 3 to about 50 atoms inlength, or about 5 to about 15 atoms in length, which are composed ofcarbon, and optionally, one or two atoms selected from O, S, NH, NR,NR′, CO, C(O)O, C(O)S, S—S, SO₂, C(O)—NR′, CS—NR′, or Si—O. Optionally,one or more of the C atoms may be substituted with a small alkyl(C₁-C₆), alkenyl, alkoxy, aryl, or diaryl groups. For example, thelinker may be an alkyl, alkenyl, or alkynyl group, optionallysubstituted as described above. In another example, the linker mayitself contain one or more O, S, NH, NR, NR′, CO, C(O)O, C(O)S, S—S,SO₂, C(O)—NR′, CS—NR′, Si—O groups bound to one or more C atoms, whichmay be optionally substituted.

In one embodiment, the linker is a structure (e.g., an alkyl group)which contains a substitution of about four to about twelve atoms with astable isotope. However, in certain embodiments, it is desirable for thelinker to contain substitutions of at least six atoms with a stableisotope. For example, for peptides at the higher end of the molecularweight range at which MS is useful (e.g., about 2000 Da to 3500 Da) itmay be desirable for the linker to contain eight, ten, twelve or moresubstitutions, in order to achieve the differential analysis required;whereas peptides at the lower end of the molecular weight range for MS(e.g., about 500 to 2000 Da) may require only four to six substitutions.For the selected number of substitutions, any one or more of thehydrogen, nitrogen, oxygen, carbon, or sulfur atoms in the linker may bereplaced with their isotopically stable isotopes: ²H, ¹³C, ¹⁵N, ¹⁷O,¹⁸O, or ³⁴S.

Thus, the linker group has a structure that accommodates the number ofisotope substitutions desired. The selection of this structure is not alimitation of the present invention. One or more of the atoms in thelinker can be substituted with a stable isotope to generate one or moresubstantially chemically identical, but isotopically distinguishablecompounds. Additionally or alternatively, the linker also optionallyprovides desired acid labile properties to the compound.

The compound of the invention further contains a functional group thatis reactive, preferably specifically, with cysteine residues. Desirably,the reactive group is selected from the group consisting of eithermaleimide (see below)

or α-haloacetyl groups such as X—CH₂CO—. Most suitably, the X isselected from halogens such as iodine, bromine, and chorine to formiodoacetyl, bromoacetyl, or chloroacetyl functionalities.

In another alternative, the thiol-reactive group may be selected fromother α-, β-conjugated double bond structures, such as

and the like. Still other reactive group can readily be synthesized tocontain other thiol-specific reactive groups for use in bindingcysteine-containing peptides.

In one preferred embodiment, a compound of the invention has theformula:

In one desirable embodiment, this compound is isotopically modified asfollows.

However, the invention is not so limited. One of skill in the art canreadily provide light ALICE with other stable isotopes. Further, one ofskill in the art can readily produce other suitable compounds in view ofthe guidance provided herein.

Method of Using the Compounds of the Invention

The compounds of the invention are particularly useful in massspectrometric methods for quantitation and identification of one or moreproteins in a mixture. The peptides analyzed by the method of theinvention are most preferably about 500 Daltons (Da) to about 3500 Da insize, but may be larger. Suitably, these peptides are formed uponenzymatic digestion of proteins in a complex mixture. The proteinmixture may be a sample from a cell or tissue culture, or biologicalfluids, cells or tissues. Samples from a culture include cellhomogenates and cell fractions. Biological fluids include urine, blood(including, e.g., whole blood, plasma and sera), cerebrospinal fluid,tears, feces, saliva, and lavage fluids. The mixtures may includeproteins, lipids, carbohydrates, and nucleic acids. The methods of theinvention employ MS and (MS)^(n) methods. Currently, matrix assistedlaser desorption ionization MS (MALDI/MS) and electrospray ionization MS(ESI/MS) methods are preferred. However, a variety of other MS and(MS)^(n) techniques may be selected.

In one embodiment, the invention provides a method for quantitativeanalysis of a proteome using the compound of the invention. Typically, asample is obtained from a source, as defined above. The sample may becompared to a reference protein mixture, which is obtained as a samplefrom the same source or may be obtained from another source. Where asample protein mixture is to be compared to a second sample or areference protein mixture, these mixtures are processed separately,applying identical reaction conditions, with the exception that onesample will be reacted with the compound containing heavy stableisotopes. Where samples are not to be compared, separate processing tothe point of reaction with the compound(s) of the invention is notnecessary, but is permitted.

Typically, the protein sample is solubilized in a suitable buffer thatmay contain an organic solvent. Throughout the entire procedure exceptthe final peptide elution step, the pH of the mixture is maintainedunder basic conditions. Most suitably, the pH is maintained between 6.5and 9, more preferably about 7.5 to 8.5, and most preferably about 7.2to 7.5.

The disulfide bonds of the proteins in the sample(s) or referencemixtures are reduced to free SH groups. Optionally, this step may becombined with solubilization of the protein or protein mixture, referredto above. Suitable reducing agents include tri-n-butylphosphine (TBP),2-mercaptoethanol, dithiothreitol, and tris-(β-carboxyethyl) phosphine.However, other suitable reducing agents may be substituted. In oneembodiment, disulfide bonds in 2 mg of a protein are denatured using 8Murea, 200 mM ammonium bicarbonate, 20 mM CaCl₂, 5 μmole TBP, which hasbeen pre-dissolved in 20 μL of acetonitrile (ACN) and incubated for onehour at about 37° C. In another embodiment, a protein may be incubatedin 50 mM Tris buffer, 6 M guanidine-HCl, 5 mM TBP at pH 8.5 for 1 hourat 37° C. However, other concentrations of these components and/or otherreducing agents, buffered to a pH in the basic range may be selected andincubated for varying lengths of times.

Free thiols (SH) are blocked using a suitable blocking reagent, e.g.,methyl methane thiosulfonate (MMTS), which functions under the basicconditions provided and does not interfere with the performance of thefollowing steps. Although MMTS is preferred, other suitable blockingreagents, including, without limitation, o-methylisourea, may beselected by one of skill in the art.

The proteins in the samples are enzymatically digested. A suitableprotease for use in this method may be readily selected from amongproteases that are compatible with the basic conditions and theprocedure. Under certain circumstances, it may be necessary to dilutethe sample mixture until any denaturing solubilizing agents in thesample are diluted to a point at which they are compatible with theactivity of the protease or proteases used. In one embodiment, theprotease is trypsin. In another embodiment, the protease is theendoproteinase Lys-C (commercially available, e.g., from Promega, RocheMolecular Biochemical). In still another example, a mixture of proteasesthat have similar activity levels at basic pH is used. Such proteasesmay include aminopeptidases, carboxypeptidases, among others.Alternatively, the protein mixture is subjected to more than onedigestion step. For example, the protein mixture may be subjected todigestion with Lys-C, followed by digestion with trypsin. Multipledigestions are particularly desirable where the mixture is a complexmixture. One of skill in the art can readily determine whether a singledigestion step, or multiple steps, are required. In yet anotheralternative, protein digestion may be omitted where the sample containspeptides, polypeptides or small proteins (e.g., about 500 to 5000 Da).

Suitably, the peptides are again reduced prior to being reacted with thecompounds of the invention to remove the blocking reagents. Thereduction step is performed using the reagents described above. In onesuitable embodiment, the mixture is reduced by incubation with 5 μmoleof TBP at 37° C. for one hour. However, other suitable concentrations,reagents, incubation temperatures and times may be readily substituted.

A selected compound of the invention and a corresponding isotopicallyheavy compound are reacted with the samples. Typically, the referencesample is labeled with the isotopically heavy compound and theexperimental sample(s) are labeled with the isotopically light form ofthe compound. However, the labeling may be reversed. Optionally, thislabeling reaction may be performed at any stage of the method, e.g.,prior to any of the reduction steps.

After completion of the tagging reaction, defined aliquots of thesamples labeled with isotopically different compounds (e.g.,corresponding light and heavy compounds) are combined and all thesubsequent steps are performed on the pooled samples. Preferably, equalamounts of each sample are pooled.

The pooled samples are washed in order to remove any non-covalentlybound species. The use of the compounds of the invention permits the useof harsher washing steps than prior art reagents can withstand. Forexample, one suitable method utilizes 5×1 mL of 50% acetonitrile (ACN),5×1 mL of 30% ACN, 5×1 mL of 90% ACN, 5×1 mL (non-diluted) ACN, and 10×5mL dichloromethane. However, the concentration of ACN may be varied.Alternatively, other suitable solvents may be substituted. Examples ofsuitable solvents include organic solvents with polarity propertiessimilar to acetonitrile or dichloromethane. Yet another suitable methodutilizes high concentrations of organic solvents, which effectivelyremoves any residual detergents or surfactants.

The tagged peptides are selectively retrieved by acid elution, whichbreaks the bond between the linker or acid labile group and the polymerto which it is covalently bond allowing the peptides tagged with thelight or heavy compounds of the invention to be eluted. For example, thelast washing may be eluted using 1% to 5% trifluoroacetic acid (TFA) indichloromethane (CH₂Cl₂). Using the method of the invention, peptiderecovery is estimated at above 75%. Suitably, recovery may be evenhigher, e.g., above 80%, 85%, and 90%, depending upon the sample andsolvents utilized.

The isolated, derivatized peptides retrieved are then analyzed using MStechniques. Both the quantity and sequence identity of the proteins fromwhich the tagged peptides originated can be determined by automatedmultistage MS. This is achieved by the operation of the massspectrometer in a dual mode in which it alternates in successive scansbetween measuring the relative quantities of peptides eluting from thecapillary column and recording the sequence information of selectedpeptides. Peptides are quantified by measuring in the MS mode therelative signal intensities for pairs of peptide ions of identicalsequence that are tagged with the isotopically light or heavy forms ofthe compounds of the invention, respectively, and which therefore differin mass by the mass differential encoded within the affinity-taggedreagent. Peptide sequence information is automatically generated byselecting peptide ions of a particular mass-to-charge (m/z) ratio forcollision-induced dissociation (CID) in the mass spectrometer operatingin the MS^(n) mode. Using computer-searching algorithms, the resultingCID spectra are then automatically correlated with sequence databases toidentify the protein from which the sequenced peptide originated. Acombination of the results generated by MS and MS^(n) analyses of thedifferentially labeled peptide samples therefore determines the relativequantities as well as the sequence identities of the components of theprotein mixtures in a single, automated operation. Alternatively, moreaccurate relative quantitation may be obtained by MS analysis of theisolated peptides with the mass spectrometer operating at MS mode only[see Automated LC/MS in Example 2: Instrumentation]

Apparatuses for performing MALDI-MS and techniques for using suchapparatuses are described in International Publication No. WO 93/24835,U.S. Pat. No. 5,288,644, R. Beavis and B. Chait, Proc. Natl. Acad. Sci.USA, 87:6873-6877 (1990); B. Chait and K. Standing, Int. J MassSpectrom, Ion Phys., 40:185 (1981) and Mamyrin et al, Sov. Phys. JETP,37:45 (1973), all of which are incorporated by reference herein.Briefly, the frequency tripled output of, e.g., a Q-switched LumonicsHY400 neodynium/yttrium aluminum garnet lawer (“Nd-YAG”) (355 nm,10-nsec output pulse) is focused by a lens (12-inch focal length)through a fused silica window onto a sample inside the massspectrometer. The product ions formed by the laser are accelerated by astatic electric potential of 30 kV. The ions then drift down a 2-m tubemaintained at a vacuum of 30 μPa and their arrival at the end of thetube is detected and recorded using, e.g., a Lecroy TR8828D transientrecorder. The transient records of up to 200 individual laser shots aresummed together and the resulting histogram is plotted as a massspectrum. Peak centroid determinations and data reduction can beperformed using a VAX workstation or other computer system. However,other apparatuses and techniques are known and may be readily utilizedfor analysis of the peptides of the invention.

Reagent Kit

The invention further provides a reagent kit for the analysis ofproteins by mass spectral analysis. Typically, such a kit will containone or more compounds of the invention. Most suitably, the kit willcontain a set of substantially identical, differentially labeled(isotopically light and heavy) compounds. In one desirable embodiment,the kit will contain the compounds of the invention such that thepolymer portion of the compound also serves as a solid support, e.g., abead or resin. The kit may further contain one or more proteolyticenzymes, blocking reagents, solubilizing detergent cocktails, or washsolutions. Other suitable components will be readily apparent to one ofskill in the art.

The method and kit of the invention may be used for a variety ofclinical and diagnostic assays, in which the presence, absence,deficiency or excess of a protein is associated with a normal or diseasestate. The method and kit of the invention can be used for qualitativeand quantitative analysis of protein expression in cells and tissues.The method and kit can also be used to screen for proteins whoseexpression levels in cells or biological fluids are affected by a drug,toxin, environmental change, or by a change in condition or cell state,e.g., disease state, malignancy, site-directed mutation, gene therapy,or gene knockouts.

The following examples are provided to illustrate the invention and donot limit the scope thereof One skilled in the art will appreciate thatalthough specific reagents and conditions are outlined in the followingexamples, modifications can be made which are meant to be encompassed bythe spirit and scope of the invention.

EXAMPLES Example 1 Synthesis of the Compound of the Invention

A. Preparation of Linker and Affinity Tag

A solution of maleic anhydride (0.98 g, 10.0 mmol in 15 ml of aceticacid) was added to a solution of 6-aminocaproic acid (1.31 g, 10 mmol in5 ml of acetic acid). The resulting mixture was stirred at roomtemperature for two hours. After two hours, the mixture was heated toreflux (oil bath temperature about 110-120° C.) for four and a halfhours. The acetic acid was removed in vacuum and 3.3 g of a light yellowsolid was obtained. This solid was chromatographed (20% ethyl acetate inhexanes, then 50% ethyl acetate in hexanes) and gave 0.92 g of puretarget compound (6-(2,5-dioxo-2,5-dihydro-pyrrol-1-yl)-hexanoic acid;43% yield). This reaction is illustrated in the scheme provided below,in which acetic acid is abbreviated as HOAc.

B. Preparation of Resin

The protected polymer, purchased commercially as NovaSyn TG Seiber resin(1 g, 0.15 mmol/g) was stirred in N,N-dimethylformamide (DMF) (8 mL) andthen piperidine (2 mL) was added. The reaction mixture was stirred forten minutes and then the solid was filtered and washed with methylenechloride and then dried under vacuum This dry solid was then againstirred with piperidine (2 mL) in DMF (8 mL) for another ten minutes.The thin layer chromatography (TLC) was recorded and showed no trace ofthe fluorenylmethyoxycarbonyl (Fmoc). The solid was then filtered andwashed with methylene chloride, dried under low pressure to give about 1g of the free amine polymer. This reaction is illustrated by thesynthetic scheme below.

The polymer is a copolymer of polyethylene glycol and polystyrene.

C. Preparation of Compound of the Invention

The deprotected polymer (1 g, 0.15 mmol/g) synthesized as described inpart B was stirred in DMF (10 mL). To this mixture was addedsequentially the compound which resulted from the reaction described inpart A (0.095 g, 0.45 mmol), 1-hydroxybenzotriazole (HOBT) (0.06 g, 0.45mmol) and N,N-dicyclohexylcarbodiimide (DCC) (0.102 g, 0.5 mmol). Thereaction mixture was stirred for three hours and the solid filtered andwashed successively with ethyl acetate, ether and methylene chloride.The solid was then dried in vacuum and gave about 1 g of the productillustrated below (ALICE of the invention).

Example 2 Instrumentation

The present invention was carried out utilizing techniques andinstrumentation known to those of skill in the art combined with a novelmethod of using the same. Specifically, data was obtained usingautomated LC/MS alone as well as using a novel automated 2-dimensionalLC/LC/MS system using instrumentation available in the art. Theseinstruments and methods of using the same are described below.

A. Automated LC/MS

Automated LC/MS was accomplished using a LC/MS MicroMass Q-ToF² massspectrometer (Micromass, Manchester, UK) equipped with an ABI 140 C.microgradient syringe pump system (Applied Biosystems, Framingham,Mass.). The sample was injected onto a strong cation exchange (SCX)column, a 100 μm×6 cm IntegraFrit column (New Objectives, Woburn, Mass.)packed with PolySULFOETHYL A, 12 μm, 300 Å (PolyLC Inc., Columbia, Md.).The sample was then eluted onto a RP-C18 column, a 75 μm×10 cm PicoFritcolumn (New Objectives, Woburn, Mass.) packed with YMC-Gel 10 μM C18beads (YMC Inc., Wilmington, N.C.) using a solution of 500 mM KCl in 0.1M acetic acid. The RP-C18 column was equilibrated with 96% aceticacid/4% ACN and then the following gradient was run: (i) 4-65% RP-B over75 minutes, (ii) 65-98% RP-B over the next 7 minutes, (iii) a hold at98% RP-B for 5 minutes, and (iv) 98-1% RP-B over the next 3 minutes at250 μL/min. Mobile-phase buffers were for RP-A: 0.1 M acetic acid, 1%ACN and RP-B: 0.1 M acetic acid, 90% ACN. Data was acquired in the MSmode only.

B. Automated 2D-LC/MSIMS

Automated 2D-LC/MS/MS was accomplished using the system as shown inFIGS. 1A and 1B. Specifically, a 2D LC-MS/MS Finnigan LCQ Deca ion trapmass spectrometer was fitted with an Applied Biosystems 140Cmicrogradient syringe pump system (Applied Biosystems, Framingham,Mass.), as the reverse phase pump (RP), and an Agilent 1100 seriesbinary pump, as the strong cation exchange (SCX) and desalting pump. Thepumps were attached to a VICI 10 port microbore two-position valve witha microelectric actuator (Valco Instruments CO Inc., Houston, Tex.). Astrong cation exchange column, 50×1 mm PolySULFOETHYL A (PolyLC Inc.,Columbia, Md.), was attached to port 9 and two 75 mm×10 cm IntegraFritcolumns (New Objectives, Woburn, Mass.) packed with YMC-Gel 10 μm C18beads (YMC Inc., Wilmington, N.C.) were attached between ports 2 and 5,and 7 and 10, respectively. Another 75 μm×3 cm C18 column packed in aPicoFrit column (New Objectives) was placed in between the titaniumvoltage union and the heated capillary of the mass spectrometer, torestore a loss of resolution from the valve and the titanium union.

Automation between the mass spectrometer, pumps and valve wasaccomplished using contact closures. First, the sample was loaded ontothe SCX column using a Rheodyne injection valve (Rheodyne, Rohnert Park,Calif.) with the port valve at position 10 as shown in FIG. 1B so thatany unbound peptides would bind to the RP-18 column and elute infraction 0. With this dual C18 column design, while one RP-C18 column(column A in FIG. 1A) is being on-line with the mass spectrometer forpeptide separation, the other C18 column (Column B in FIG. 1A) is beingregenerated, loaded with peptide sample eluted from the SCX column anddesalted. After each HPLC gradient run is completed, the positions ofthe two RP-C18 columns were switched over using the two-positionten-port valve (FIG. 1B) so that the time delay for equilibrating,sample loading from SCX and desalting was effectively eliminated.Peptide factions were eluted from the SCX column onto one RP-C18 columnusing the following salt steps: (i) 5%, (ii) 10%, (iii) 15%, (iv) 20%,(v) 30%, (vi) 40%, (vii) 50%, (viii) 65%, (ix) 85%, (x) 98%, (xi) 98%,(xii) 98%, and (xiii) 98%, SCX-B:SCX-A, for 10 minutes at 1 μL/min.Before each elution, 100% SCX-A was flowed at 1 μL/min for 20 minutes toequilibrate the RP C18 column and after each salt elution, 100% SCX-Awas flowed at 1 μL/min for 20 minutes for elutions (i) to (iv), 25minutes for elutions (v) and (vi), 30 minutes for elutions (vii) and(viii), and 35 minutes for elutions (ix) to (xiii). The flow was thenslowed down to 200 nL/min for the remainder of time to rinse the saltfrom the RP C18 column. Peptides were eluted from one C18 column intothe mass spectrometer using a linear RP gradient: a) 1-65% RP-B over 75minutes, b) 65-98% RP-B over the next 7 minutes, c) a hold at 98% RP-Bfor 5 minutes, and d) 98-1% RP-B over the next 3 minutes at 400 nL/min.Mobile-phase buffers were, RP-A: 0.1 M acetic acid, 1% ACN; RP-B: 0.1 Macetic acid, 90% ACN; SCX-A: 0.1 M acetic acid, 1% ACN; SCX-B: 500 mMKCI. (FIGS. 1A and 1B).

Example 3 Preparation of Proteomes for MS Analysis

2 mg of bovine serum albumin (BSA) were solubilized in 200 μL of 8 Murea, 200 mM ammonium bicarbonate, and 20 mM CaCl₂. 5 μmole of tributylphosphine (TBP) pre-dissolved in 20 μL of acetonitrile (ACN) was addedinto the solubilized protein mixture and the resulting solution wasincubated at 37° C. for one hour. To the protein mixture was added 11μmoles of MMTS and the mixture was vortexed for 10 minutes. The proteinsolution was diluted 1:1 with 100 mM ammonium bicarbonate and 40 μg ofLys-C (2% w/w) were added. This mixture was then incubated at 37° C. for5 hours. The resulting solution was diluted 1:1 with water and thenproteins were further digested with trypsin (2% w/w) at 37° C. for 15hours. The resulting peptide solution was dried and then reconstitutedwith 50% acetonitrile/200 mM sodium phosphate (pH 7.2). Disulfide bondson the cysteine-containing peptides were reduced with TBP (5 μmoles) at37° C. for one hour. Then 50 mg of the ALICE resin (about 11.5 μmolereactive sites) was added into the peptide solution and the solutionvortexed for 1 hour at room temperature. The solutions were combined andloaded onto a column (glass type with teflon cockstop) and the resin waswashed with the following solvent in sequence: 1) 5×1 mL of 50% ACN, 2)5×1 mL of 30% ACN, 3) 5×1 mL of 90% ACN, 4) 5×1 mL of pure ACN, 5) 10×5mL of dichloromethane (DCM).

Cysteine-containing peptides were then eluted from the resin with 5% TFAin DCM using continuous flow methodology. The resulting peptide solutionwas dried and reconstituted with 1% acetic acid in water. Thereconstituted peptide solution was directly subjected to automated2D-LC/MS/MS analysis (as described above) without further treatment. MSanalysis combined with database searching yielded both identities andquantities of the proteins.

Samples were taken from the mixture before and after acid elution for MSanalysis to compare the overall recovery of cysteine-containing peptideswith or without using the ALICE approach. The results are providedbelow, with reference to the following published sequence of bovineserum albumin (using single letter amino acid code):   1 MKWVTFISLLLLFSSATYSRG VFRRDTHKSE IAHRFKDLGE SEQ ID NO. 1  41 EHFKGLVLIAFSQYLQQCPF  DEHVKLVNEL TEFAKTCVAD  81 ESHAGCEKSL HTLFGDELCK  VASLRETYGDMADCCEKQEP 121 ERNECFLSHK DDSPDLPKLK  PDPNTLCDEF KADEKKFWGK 161YLYEIARRHP YFYAPELLYY  ANKYNGVFQE CCQAEDKGAC 201 LLPKIETMREKVLTSSARQR  LRCASIQKFG ERALKAWSVA 241 RLSQKFPKAE FVEVTKLVTD  LTKVHKECCHGDLLECADDR 281 ADLAKYICKN QDTISSKLKE  CCDKPLLEKS HCIAEVEKDA 321IPENLPPLTA DFAEDKDVCK  NYQEAKDAFL GSFLYEYSRR 361 HPEYAVSVLLRLAKEYEATL  EECCAKDDPH ACYSTVFDKL 401 KHLVDEPQNL IDQNCDQFEK  LGEYGFQNALIVRYTRKVPQ 441 VSTPTLVEVS RSLGKVGTRC  CTKPESERMP CTEDYLSLIL 481NRLCVHEKT  PVSEKVTKCC  TESLVNRRPC FSALTDETY 521 VPKAFDEKLFTFHADICTLP  DTEKQIKKQT ALVELLKHKP 561 KATEEQLKTV MENFVAFVDK  CCAADDKEACFAVEGPKLVV 601 STQTALA:

Peptides identified from peptide mixtures before and after using ALICEfor isolation of cysteine-containing peptides Peptides identified byLC-MS/MS and database searching Peptides identified by from the sampleafter LC-MS/MS and database enzymatic digestion but searching from thefinal before reaction with ALICE sample eluted from the (including bothcysteine ALICE resin (exclusively containing and non-cysteinecysteine-containing containing peptides) peptides) Position, Position,based on SEQ ID NO. 1 based on SEQ ID NO. 1* 508-523 76-88 508-523460-468 402-412 437-451  89-100 483-489 106-117  89-100 267-280 123-130198-204 298-309 106-117 286-297 310-318 267-280 581-587 499-507 161-167375-386 45-65 199-204 123-130 499-507 310-318 198-204 286-297 360-37176-88 300-309 460-468 562-568 588-597 387-399 421-433 123-138 52-65375-386 529-544  95-100 139-151 319-340 300-309 588-597 413-420 223-228413-420 533-544 529-544 469-482 598-607 548-557 35-44 172-183 45-65319-340 347-359 469-482 341-353 435-451 354-359 413-424 168-180 387-399361-371 66-75 581-597 549-557 569-580 139-151*Two highlighted cysteine-containing peptides: CASIQK (residues 223-228)and LCVLHEK (residues 483-489) were only detected from the final sampleeluted from the ALICE resin.

This study demonstrated that nonspecific binding associated with the useof conventional reagents is not a problem using the compounds of theinvention, since all the peptides eluted from the resin after washingare exclusively cysteine-containing peptides. This is because thecompounds of the invention permit the use of much more stringent washingconditions, as compared to conventional ICAT reagents. Thus, thecompounds of the invention provide lower “noise”, better dynamic rangeand sensitivity in subsequent MS analysis.

More specifically, in this study, 33 out of 35 cysteines were captured.Only one Cys-containing peptide, YNGVFQECCQAEDK (residues 184-197 of SEQID NO. 1) was not recovered either before or after isolation CASIQK(residues 223-228 of SEQ ID NO. 1) and LCVLHEK (residues 483-489 of SEQID NO. 1) were only seen after isolation. This is likely due to thebetter dynamic range and sensitivity provided by the compound of theinvention. Although not measured, overall recovery percentage isanticipated to be more than 75%. Steric hindrance in the capturing stepis not a problem, since the peptides containing more than one cysteinewere all uniformly modified by ALICE, the model compound of theinvention. From all the CID experiments, no fragments observed were fromthe ALICE label, indicating that the compound would not interfere withthe MS/MS experiments and subsequent protein identification byfragment-ion based database searching.

Example 4 Capturing Cysteine-Containing Peptides Using Alice, SimpleProtein Mixtures, and Automated LC/MS and 2D-LC/MS

Two mixtures were prepared, each containing eight proteins. Thefollowing table illustrates the composition of these mixtures.Composition of two protein mixtures Protein Mixture Protein MixtureProtein Name A (nmol) B (nmol) Lysozyme 10 50 α-lactalbumin 50 10Ovalbumin 25 50 Catalase 50 25 β-lactoglobulin 38 50 BSA 50 38Ribonuclease 50 50 Trypsinogen 50 50

Protein mixture A and protein mixture B (323 nmol of total protein weresolubilized, respectively, in 325 μL of 6 M urea, 5%3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonate (CHAPS), and50 MM Tris HCl. 11.3 μmole of tributyl phosphine (TBP) pre-dissolved in6.3 μL of isopropanol (IPA) was added to each solubilized proteinmixture and the resulting solutions were incubated at 37° C. for onehour. To each protein mixture was added 200 μL of 50mM Tris-HCI (pH 8.0)and 34 μmol of methanethiosulfonate (MMTS) predissolved in 3.5 μL ofIPA, and the mixtures were reacted for 30 minutes. Each protein solutionwas diluted four times with 50 mM Tris-HCI (pH 8.0) and digested withtrypsin (5% w/w) at 37° C. for 16 hours. From the total peptidemixtures, 42% (21% from each mixture) was retained for future work, andthe remaining 58% (187 nmol total protein) was dried and thenreconstituted with 1.5 mL of 60% acetonitrile (ACN)/40% 100 mM Tris-HCI(pH 7.0). Disulfide bonds on the cysteine-containing peptides werereduced by TBP (18.7 μmol) at 37° C. for one hour. Each solution wasthen vacuum concentrated for 10 minutes to remove excess TBP and ACN,and reconstituted to the previous volume using ACN. To each solution wasadded 55 μmol of either light or heavy ALICE resins (3×TBP molarequivalent) and the solutions were stirred for 1 hour at roomtemperature. The reactions were quenched by the addition ofP-mercaptoethanol (BME) to a final concentration of 1%.

The protein mixtures were then combined and loaded onto a column(fritted glass type with Teflon cockstop) and the resin was washed withthe following solvent in sequence: (i) 50 mL of a 50:50 ACN:watersolution, (ii) 50 mL of pure ACN, (iii) 50 mL of a 50:50ACN:dichloromethane (DCM) solution, and (iv) 50 mL of pure DCM.

Cysteine-containing peptides were isolated by elution with 3×5 mL of 5%TFA in DCM using continuous flow methodology, 15 minute incubations withintermittent shaking, then 15 mL of continuous flow. The resultingpeptide solution was dried and reconstituted with 2% ACN in 1% aceticacid/water. The reconstituted peptide solution was directly subjected toHPLC-MS MicroMass Q-ToF² instrument (MicroMass, Manchester, UK) and2D-LC-MS/MS (Finnigan LCQ Deca, Finnigan Corporation, San Jose, Calif.)analysis without further treatment. These analyses, combined withdatabase searching, yielded both identities and quantities of theproteins. The chemical reactions for the isolation ofcysteine-containing peptides are illustrated in the following scheme.

The results of the mass-spectrometric analysis are provided in thefollowing table. In this table, M#=oxidized methionine residue; C*=lightand heavy ALICE labeled cysteine residue. TABLE Sequence identificationand quantitation of the components of a protein mixture using ALICE.Peptide Mass// Obs'd Charge Peptide Sequence Ratio/ Exp. % Protein NameState identified/SEQ ID NO: Mean ± SD Ratio Error α-lactoalbumin 432.20//2 (K)C*EVFR(E) 4.97 5 0.6 SEQ ID NO: 3 β-lactoglobulin/1107.84//3 (K)YLLFC*M#ENSAEPEQSLVC*QC*LVR(T): 0.76 0.76 0.3 SEQ ID NO: 40.76 ± 0.01  934.94//2 (R)LSFNPTQLEEQC*HI(-): 0.77 SEQ ID NO: 5 Catalase 654.34//2 (R)LC*ENIAGHLK(D) 2.1  2 1 SEQ ID NO: 6 0.02 ± 0.09 436.56//3 (R)LC*ENIAGHLK(D): 1.93 SEQ ID NO: 6  979.00//2(R)LGPNYLQIPVNC*PYR(A): 2.01 SEQ ID NO: 7 Lysozyme 1062.49//1(R)C*ELAAAM#K(R): 0.2  0.2 0.2 SEQ ID NO: 8 Ovalbumin  739.80//2(A)SM#EFCFDVFK(E): 0.61 0.5 16 SEQ ID NO: 9 0.58 ± 0.05  700.85//2(R)ADHPFLFC*IK(H): 0.6  SEQ ID NO: 10  467.57//3 (R)ADHPFLFC*IK(H): 0.52SEQ ID NO: 10  838.44//2 (R)YPILPEYLQC*VK(E) 0.59 SEQ ID NO: 11Ribonuclease 1189.08//2 (K)HIIVAC*EGNPYVPVHFDASV(-) 1.08 1 0.4 SEQ IDNO: 12 1.00 ± 0.11  793.06//3 (K)HIIVAC*EGNPYVPVHFDASV(-) 1.16 SEQ IDNO: 12  595.04//4 (K)HIIVAC*EGNPYVPVHFDASV(-) 1.17 SEQ ID NO: 12 706.60//4 (R)C*KPVNTFVHESLADVQAVC*SQK(N) 0.89 SEQ ID NO: 13  922.40//2(A)CEGNPYVPVHFDASV(-) 1.03 aa 6-22 of SEQ ID NO: 12  608.63//3(F)VHESLADVQAVCSQK(N) 0.96 aa 6-24 of SEQ ID NO: 12   865.5//1(K)HIIVAC*(E) 1.03 aa 1-8 of SEQ ID NO: 14  433.25//2 (K)HIIVAC*(E) 0.9 aa 1-8 of SEQ ID NO: 14  1239.5//1 (Y)STM#SITDC*R(E) 0.9  SEQ ID NO: 14 620.25//2 (Y)STM#SITDC*R(E) 0.84 SEQ ID NO: 14 Trypsinogen   580.3//2(A)PILSDSSC*K(S) 0.87 1 2 aa 5-15 of SEQ ID NO: 15 1.02 ± 0.101230.61//1 (K)APILSDSSC*K(S) 1.01 aa 4-15 of SEQ ID NO: 15  615.80//2(K)APILSDSSC*K(S) 1.18 aa 4-15 of SEQ ID NO: 15  892.95//2(K)C*LKAPILSDSSC*K(S) 1.02 SEQ ID NO: 15  595.63//3(K)C*LKAPILSDSSC*K(S) 1.04 SEQ ID NO: 15  958.41//2(K)DSC*QGDSGGPVVC*SGK(L) 0.98 SEQ ID NO: 16 BSA  1141.6//1(C)C*TESLVNR(R) 1.5  1.32 2.3 aa 497-506 of SEQ ID NO: 1 1.35 ± 0.10 566.25//2 (C)C*TESLVNR(R) 1.28 aa 497-506 of SEQ ID NO: 1  623.35//2(H)TLFGDELC*K(V) 1.21 aa 92-102 of SEQ ID NO: 1 1194.02//2(K)C*C*AADDKEAC*FAVEGPK(L) 1.24 aa 577-595 of SEQ ID NO: 1  796.35//3(K)C*C*AADDKEAC*FAVEGPKQ(L) 1.23 aa 577-595 of SEQ ID NO: 1  722.83//2(K)C*C*TESLVNR(R) 1.34 aa 496-506 of SEQ ID NO: 1  650.30//3(K)DDPHAC*YSTVFDKLK(H) 1.35 aa 386-402 of SEQ ID NO: 1  630.80//2(K)EAC*FAVEGPK(L) 1.3  aa 584-595 of SEQ ID NO: 1  533.25//3(K)EC*C*DKPLLEK(S) 1.41 aa 300-311 of SEQ ID NO: 1  911.50//1(K)GAC*LLPK(I) 1.48 aa 198-206 of SEQ ID NO: 1  638.80//2(K)LFTHADIC*(T) 1.35 aa 525-535 of SEQ ID NO: 1  638.80//2(K)LFTFHADIC*(T) 1.51 aa 525-535 of SEQ ID NO: 1  613.65//3(K)LKEC*C*DKPLLEK(S) 1.51 aa 298-311 of SEQ ID NO: 1  577.28//3(K)LKPDPNTLC*DEFK(A) 1.21 aa 139-153 of SEQ ID NO: 1  786.89//2(K)SLHTLFGDELC*K(V) 1.35 aa 89-102 of SEQ ID NO: 1  524.92//3(K)SLHTLFGDELC*K(V) 1.35 aa 89-102 of SEQ ID NO: 1  885.37//2(K)TC*VADESHAGC*EK(S) 1.52 aa 76-90 of SEQ ID NO: 1  590.58//3(K)TC*VADESHAGC*EK(S) 1.52 aa 76-90 of SEQ ID NO: 1  591.62//3(K)VTKC*C*TESLVNR(R) 1.19 aa 493-506 of SEQ ID NO: 1  798.86//2(K)YIC*DNQDTISSK(L) 1.36 aa 286-299 of SEQ ID NO: 1 1027.43//2(K)YNGVFQEC*C*QAEDK(G) 1.2  aa 184-199 of SEQ ID NO: 1  859.43//1(R)C*ASIQK(F) 1.46 aa 223-230 of SEQ ID NO: 1  430.21//2 (R)C*ASIQK(F)1.3- aa 223-230 of SEQ ID NO: 1 1051.56//1 (R)LC*VLHEK(T) 1.35 aa481-488 of SEQ ID NO: 1 526.284//2 (R)LC*VLHEK(T) 1.27 aa 481-488 of SEQID NO: 1  947.45//2 (R)M#PC*TEDYLSLILNR(L) 1.36 aa 468-482 of SEQ ID NO:1  631.97//3 (R)M#PC*TEDYLSLILNR(L) 1.25 aa 468-482 of SEQ ID NO: 11027.97//2 (R)NEC*FLSHKDDSPDLPK(L) 1.27 aa 123-140 of SEQ ID NO: 11017.50//2 (R)RPC*FSALTPDETYVPK(A) 1.41 aa 505-521 of SEQ ID NO: 1678.672//3 (R)RPC*FSALTPDETYVPK(A) 1.39 aa 505-521 of SEQ ID NO: 1

This study demonstrated that quantification by ALICE is accurate aftertaking into account the following factors: isotopic impurity of theheavy ALICE; different elution profile of the same peptides modified byheavy and light ALICE; non-specific enzymatic cleavage. This improvedquantitation accuracy by ALICE is even more evident when multiplecysteine-containing peptides are present. Peptides without any cysteineresidue were rarely seen in the final captured peptide mixture sincemore stringent washing conditions completely removed non-specificallybound species. Furthermore, the use of large amounts of organic solventsalso minimized the loss of peptides throughout the procedure. Finally,simplification of the peptide mixture by isolating cysteine-containingpeptides in combination with the novel automated 2D-LC/MS designincrease the overall sample loading capacity, the speed of sampleanalysis and the dynamic range and sensitivity of the MS analysis ofprotein mixtures. This experiment also further confirmed that reactionbetween ALICE and cysteine-containing peptides is efficient andstoichiometric and the effect of steric hindrance is not a concern sincepeptides with more than one cysteine residue were modified completely byALICE. For example, a tryptic peptide with three cysteine residuesderived from lysozyme (NLC*NIPC*SALLSSDITASVNC*AK, SEQ ID NO:2) wasuniformly labeled with either heavy or light ALICE (the mass difference(not shown) between this heavy and light mass pairs is exactly 30 Da).Both light and heavy ALICE labeled peptides were effectively picked bythe automated 2D-LC/LC/MS system for MS/MS analysis even though the peakintensity for the light ALICE labeled peptide is very low. Subsequentdatabase searching identified the peptide as NLC*NIPC*SALLSSDITASVNC*AK[SEQ ID NO:2] with cysteine residues modified by light and heavy ALICE,respectively.

All publications cited in this specification are incorporated herein byreference herein. While the invention has been described with referenceto a particularly preferred embodiment, it will be appreciated thatmodifications can be made without departing from the spirit of theinvention. Such modifications are intended to fall within the scope ofthe appended claims.

1-15. (Canceled).
 16. A compound useful for capturing cysteine-containing peptides, which is selected from the group consisting of a thiol-specific reactive group attached to a non-biological polymer via a linker.
 17. The compound according to claim 16, wherein the linker contains a substitution of at least six atoms with a stable isotope.
 18. The compound according to claim 16, wherein the linker contains ten stable isotopes.
 19. The compound according to claim 17, wherein the stable isotope is deuterium.
 20. The compound according to claim 16, selected from the group consisting of:


21. A reagent kit for the analysis of proteins by mass spectral analysis that comprises a compound of claim
 16. 22. The reagent kit of claim 21 which comprises a set of substantially identical differentially labeled cysteine-tagging reagents.
 23. The reagent kit of claim 22 further comprising one or more proteolytic enzymes for use in digestion of proteins to be analyzed. 